1. Field of the Invention
This invention concerns temperature measurement of earth formations. More specifically, it relates to method and apparatus for measuring temperature of an earth formation in the presence of a radio frequency electromagnetic field.
2. Description of the Prior Art
There is an arrangement shown in U.S. Pat. No. 2,633,025 to E. G. Boice et al dated Mar. 31, 1953. However, that patent provides a thermometer mounted in a drill bit, and it employs a specially constructed mercury thermometer which permits the mercury column to expand out of its restricted passage leaving only the mercury from the maximum temperature. Thereafter, in order to determine the temperature at that maximum, the special thermometer must be reheated along with an indicating thermometer so that when the mercury column just reaches the exit of the special thermometer, the temperature of the other (indicating) thermometer may be read. This involves a cumbersome procedure which means that the maximum temperature found by the special thermometer is not readily determined after it has been returned to the surface.
Another prior U.S. Pat. No. 4,227,404 to Jerry B. West dated Oct. 14, 1980, discloses a complex equipment arrangement which includes measurement of temperature down a borehole. However, the determination of temperature is only one of a large number of other sensors used for determining different data. And, the arrangement includes a complex tool which has DC power lines for transmitting the data from the tool located down hole, to the surface. Clearly, the arrangement includes electrical conductors in a cable that extends from the surface to the logging tool. And, it would be unacceptable for use with the method and system according to the applicants invention, since it would distort radio frequency fields that are existing during the temperature measurements according to applicants invention.